A new phenomenon of surface bilayer formation has been observed, and the conditions where it forms have been identified. This phenomenon is part of the general class of azeotropic transitions. The dependence of the temperature of surface bilayer formation on the chemical nature; of the phospholipid appears to parallel the variation of the phospholipid composition of bacterial membranes with growth temperature. This suggests that the physical processes which determine the temperature of formation of surface bilayers may be operating when lipids are incorporated into cell membranes during cell growth. Disaturated lecithin titer in blood plasma has been measured in a population that is free of the major risk factors for coronary artery disease. These have been contrasted with patients with atherosclerosis, as identified by coronary angiograms. A lower limit of the lecithin titer has been tentatively identified as 1.4 mg/dl, below which it is believed that atherosclerosis is not likely to occur.